bpf: Reject non-negative offsets in stack_slot_obj_get_spi()

Overview

In the Linux kernel, the following vulnerability has been resolved:

bpf: Reject non-negative offsets in stack_slot_obj_get_spi()

bpf_get_spi() computes (-off - 1) / BPF_REG_SIZE using C division, which truncates toward zero. For off == 0, this produces spi 0, the same index used by the valid stack slot at fp-8.

stack_slot_obj_get_spi() currently checks alignment and the resulting spi bounds, but does not reject the non-negative offset itself. It can therefore validate a PTR_TO_STACK register holding fp+0 against an iterator stored at fp-8 even though the runtime receives the actual fp+0 pointer. An effectful iterator kfunc can then interpret memory outside the BPF stack as iterator state.

Reject non-negative offsets before converting the offset to an spi. All valid stack objects begin at a negative offset from the frame pointer.

Affected

• Linux >= 06accc8779c1d558a5b5a21f2ac82b0c95827ddd < 89a7241fc7ecc1b5e47c3f071081e314523b8d9d
• Linux >= 06accc8779c1d558a5b5a21f2ac82b0c95827ddd < 79a9172f3ab4ad8392c5e5c8944b9b7710ade620
• Linux 6.4

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.